Anthropogenic Storms in the Baltic Sea and the Coastal Response

ANTHROPOGENIC STORMS IN THE BALTIC SEA AND THE COASTAL RESPONSE Victor Alari Marine Systems Institute at Tallinn University of Technology Akadeemia Rd 21b, 12618, Tallinn, Estonia Email: [email protected] Ants Erm Marine Systems Institute at Tallinn University of Technology Urmas Raudsepp Marine Systems Institute at Tallinn University of Technology 1. INTRODUCTION The Baltic Sea is nearly tideless (amplitudes of M2 and K1 waves are 0.01–0.02 m in the Baltic Proper, e.g. Suursaar et al, 2003), a large brackish inland sea, located in north- eastern Europe (Fig. 1). 16 million people in 9 countries residue near the Baltic Sea coast, therefore making the sea an important area for food, recreation and transport. Due to the complex geometry with straits, sills, archipelagos and open sea areas, the Baltic Sea can be regarded as a number of natural sub-basins (Omstedt and Axell, 2003). The Gulf of Finland is a sub-basin in eastern part of the Baltic Sea (Fig. 1). It is an elongated estuarine sea with an open boundary to the west. Two capital cities lie at the opposite coasts of the Gulf of Finland: Helsinki in the north and Tallinn in the south. Between these cities, vessel traffic has rapidly increased after the collapse of the Soviet Union. Waves generated by high speed crafts (HSC) in the vicinity of ship lanes have become a problem of growing concern in many countries (Kofoed-Hansen and Mikkelsen, 1997; Parnell and Kofoed-Hansen, 2001; Velegrakis et al, 2007), among them Estonia, and particularly in Tallinn Bay. High-speed vessels (monohulls, hydrofoils and catamarans) with cruising speeds up to 40 knots (kn) were operating the Tallinn–Helsinki route between 1999 and 2008, whereas the most intense traffic occurred in 2005 (Erm et al, 2008), when 45 crossings per day was counted. Tallinn Bay is a small sea area located in the southern Gulf of Finland (Fig. 2). It is a confined bay protected from waves due to the shallows and islands, whereas only certain wind conditions can lead to a high wave activity in the bay interior (Soomere, 2005). Intense traffic of high speed crafts has led to a situation, where ship wakes form an essential component of the hydrodynamic activity on some sections of the coast of Tallinn Bay. From 2009, the fleet of vessels which induce high wakes are characterized by long (~ 200 m), highly powered crafts with service speeds of up to 27.5 knots (Table 1). Nowadays, these vessels make 16 crossings daily, almost every day (sometimes, on Sundays they rest). With the use of wave measurements, underwater light field measurements and numerical modelling, it is the purpose of this study to characterize the unique properties of the wakes from high speed crafts and interactions with the sea bed in the coastal regions of Tallinn Bay. The paper is structured as follows: an overview of the ship park and the theory of generation of vessel wakes, together with the measurements of waves and optics, modelling, are presented in Section 2. Section 3 demonstrates the properties of the waves both of anthropogenic and natural origin, and the impact of the wakes on the sediment resuspension and underwater light field. The results are discussed in Section 4 together with methods minimizing the hazardous effects of fast ferry wakes. The work is concluded in Section 5. Figure 1. The Baltic Sea and the approximate location of Tallinn Bay (black arrow). Table 1. The vessels and their characteristics. F h and F l denote the depth and length Froude numbers, respectively. *the vessels are no longer in service. Length Width Draught Speed Fh 30 Fh 40 Fh 50 Fh 60 Fl, max Vessel Type (m) (m) (m) (kn) m m m m speed HSC SuperSeaCat (SSC)* Monohull 100,3 17,5 2,6 35 1,05 0,91 0,81 0,74 0,57 HSC Nordic Jet (NJL)* Catamaran 60 16,5 2,22 36 1,08 0,93 0,84 0,76 0,76 Twin hull HSC Merilin hydrofoil 52 13 1,51 40 1,20 1,04 0,93 0,85 0,91 M/S Star Monohull 186 27,7 6,8 27,5 0,82 0,71 0,64 0,58 0,33 M/S SuperStar Monohull 177 27,6 7,1 27,5 0,82 0,71 0,64 0,58 0,34 M/S Viking XPRS Monohull 185 27,7 6,5 25 0,75 0,65 0,58 0,53 0,30 M/S Superfast Monohull 203 25 6,5 25 0,75 0,65 0,58 0,53 0,29 Figure 2. Tallinn Bay. The magenta line represents the outgoing fairway and the green line the incoming fairway. (•) is the Vanasadam harbour, ( *) the Katariina Jetty, ( x) the measurement site near Aegna Island and ( •) the place where wind wave statistics is extracted. 2. DATA AND METHODS 2.1 GENERATION OF WAKE WAVES. THE FLEET Every vessel making way generates perturbations in the bow and stearn region. The physical mechanism behind the generation of vessel wakes is as follows. The distribution of the hydrodynamic forces along the hull is not constant. A high pressure region exists in the bow and at the aft, whereas a low pressure system is aligned at midship. This results in the curving of the free water surface. Where the flux of the pressure is greater than the atmospheric pressure, wave crests form. Where it is lower than air pressure, wave through appear. Water particles, which are forced away from their equilibrium state, now try to reach their original position due to inertial and gravity forces. This result a floating process, which is seen for a naked eye as vessel wakes (diverging and transverse), forming on the relaxed water surface. Thousands of vessels are sailing in the coastal region at every time moment. But not all of them generate wakes, which are potentially hazardous to coasts and other vessels, as well as for humans. The generation of high, long-crested asymmetrical waves, need special preconditions. These preconditions are usually described in the literature with the depth and length Froude numbers. For example, high speed vessel can be interpreted as a vessel, which has a regular sailing regime of depth Froude number over 0.6 (this is the limit where the normal Kelvin-Havelock wave pattern is modified and special non-linear features may appear) and the length Froude number between 0.4-0.6. The dimensionless length and depth Froude numbers are defined, respectively: = U FL gL = U FH , gh where U is the ships speed, g is the acceleration due to gravity, L is the ship length and h is the water depth. The vessel fairway is located in the central part of Tallinn Bay and passenger ships depart and arrive in the Vanasadam harbour (Fig. 2). The outgoing vessels use eastern part of Tallinn Bay and incoming vessels use western part of the Tallinn Bay. While the incoming leg moves with the bottom slope, the outgoing leg is rather parallel to the coastal slope of eastern part of Tallinn Bay. Quite long section of this is aligned with the 40 m isobaths. As the aim of this study is to describe the impact of wakes from crafts capable of moving with high depth Froude numbers, the conventional ferries with sailing speeds of order 20 kn is not listed in table 1. The conventional ferries sail at moderate depth Froude numbers ( ≤ 0.5) and do not generate waves higher than 30 cm and periods 4 s in Tallinn Bay. 2.2 MEASUREMENTS OF WAVES AND UNDERWATER LIGHT FIELD Although the ship wakes cross Tallinn Bay and could reach even the Naissaar Island, which is some 8 km west of the incoming fairway, the effect of wakes there is negligible, as the wakes are dissipated on the shallows east of Naissaar. The northbound sailing vessels posses a threat to eastern part of Tallinn Bay and on southern coasts of the Aegna Island (as the apex angle of Kelvin wedge is about 30 o for the diverging wakes which hit Aegna Island; Torsvik et al, 2009). The vessels sailing southbound generate waves, which affect the ports located in southern part of Tallinn Bay (especially the Lennusadam harbour, where the wake induced harbour resonance tears loose ships lines) and a famous recreational area the Pirita beach. Extensive measurement campaigns have been made at southern coast of Aegna Island during recent years. The wave measurements were usually conducted with pressure based sensors with a sampling frequency of 4 Hz. When the background of wind waves was necessary, the signal was corrected with the frequency dependent attenuation coefficient. Simultaneously with the wave measurements, an optical sonde was also located in the bottom in order to quantify the impact of waves on sediment resuspension. The parameter that describes the worsening of light conditions is the diffuse attenuation coefficient of irradiation. In this study the main attention is on the measurements conducted in July 2008 near Aegna coast, as that recording gives us six days of wind-wave free environment (mean wind speed was ~3 m s -1 during this period) with a high prevalence of ship wakes. A two day long measurement campaign was also conducted in 2009 near the Katariina Jetty (Fig. 2), along with the measurements of underwater light conditions. This record is not affected by waves from HSC SSC and HSC NJL , as they were not operating anymore in 2009. A record from 2006 is also used to show, how immense a non-breaking wake wave can be in 5 m of depth.

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